The structure of one of the basic members of the cell-membrane water-channel family, a protein called aquaporin 1 (AQP1), has been determined to a resolution of 2.2 angstroms (22 billionths of a meter).
The structure reveals the elegantly simple means by which AQP1 can transport water through the cell membrane at a high rate while effectively blocking everything else, even individual protons, the nuclei of hydrogen atoms.
Biophysicist Bing Jap led a team from Lawrence Berkeley National Laboratory's Life Sciences Division in the difficult and painstaking crystallization of this membrane protein, whose crystal structure was then solved from x-ray diffraction at Beamline 5.0.2 of Berkeley Lab's Advanced Light Source. Their report appears in the journal Nature today.
In almost all cells, from bacteria to those found in a variety of human tissues, pores embedded in cell membranes transport water rapidly into or out of the cell. Body temperature, digestion, reproduction, fluid pressure in the eye, and water conservation in the kidney are only a few of the processes in humans that depend on the proper functioning of cellular water channels.
"Membrane proteins are a very large class of proteins; some 30 percent of the genes in the human genome code for them. But they are notoriously difficult to crystallize, and only a few structures have been solved at very high resolution," Jap says.
Electron-microscope crystallography can use very small crystals, and the structure of AQP1 had previously been solved to a resolution of about 4 angstoms using this technique. At this resolution it is impossible to see individual water molecules, however, so vital features were left out or mistakenly characterized.
Jap and his colleagues crystallized AQP1, closely similar to that in human and other cells, from bovine red blood cells. They liberated enough protein from "gallons of blood" to make .2 millimeter crystals, suitable for x-ray crystallography at the Advanced Light Source.
"AQP1 is interesting because it is so specific for water" says Jap. "The key question was how it achieves this specificity. Theorists had come up with lots of ideas, but before we saw the structure in high resolution, nobody knew how it was accomplished."
Architecturally, AQP1 is an assembly of four units, each with three major structural features: each has an entrance, or "vestibule," on the outside of the cell envelope, connected to a similar vestibule inside the cell by a long, narrow pore.
"The secret of AQP1's specificity is two-fold: it selects for size and for chemical nature," Jap says. "There is a very narrow constriction in the pore, which admits no molecule bigger than water. To keep out molecules smaller than water there is also a chemical filter, formed by the specific orientation and distribution of the amino acid residues lining the pore."
Molecules attempting to enter the channel are bound to water molecules that are stripped away in the pore; charged species are therefore left with net electrical charge. "The filter strongly rejects charged molecules or ions, even as small as single protons," Jap explains.
The unique distribution of amino acid residues along the pore wall also accounts for the channel's ability to move water quickly, explains Peter Walian, a member of the team that solved the structure. "It's a schizophrenic environment, half hydrophilic and half hydrophobic," that is, half water-loving and half water-fearing. "Water molecules readily get in because of the hydrophilic sites, but the hydrophobic regions prevent them from binding too frequently."
Thus water and only water flows freely in and out of the cell through AQP1's pores, the direction of flow depending only on changing relative pressure inside and outside the cell. "It's a beautiful mechanism," Walian remarks. "It's remarkable that nobody thought of it before now."
"This is what structural biology is for," Jap says. "It shows us how extremely simple nature's solutions can be."
"Structural basis of water specific transport through AQP1 water channel," by Haixin Sui, Bong-Gyoon Han, John K. Lee, Peter Walian, and Bing K. Jap, appears in today's Nature.
Images of the AQP1 structure can be reached from the Berkeley Lab home page.
The Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California. - By Paul Preuss
[Contact: Paul Preuss]